Which Kingdom Hearts games should I play?

you can kind of get by not playing chain of memories, but it'll be a bit confusing since kh2 happens directly after it. the main antagonists, the opening, the enemies, and even important parts of the plot occur in chain of memories. if you don't like the card game play, just watch the cut scenes on youtube or something. games like recoded, 358/2, and birth by sleep are prequels; they more describe other things in the games like why riku looks a certain way, why they can use keyblades, who ansem really is, etc. but there's a youtube video called "kingdom hearts, the story so far" that you can watch in place of playing all the games. it's a very complicated story line, i should warn you. but bottom line is the only game that should really be understood before playing kh2 is chain of memories and kh1.

Next play Birth by Sleep for PSP. Prequel to the whole shebang.

Yeah you kind of do COM is important for the opening of KH2.